1. Field of the Invention
The present invention relates to an improved method for the production of nodular cast iron.
2. Description of the Prior Art
The carbon present in molten cast iron is normally present in the so-called flake form. If the metal is solidified in this form, however, the cast iron produced has properties which are inappropriate for certain uses, in particular, it has low hardness and tensile strength properties. It has, therefore, been known for some years to convert the flake to graphite to nodular form by treating the grey iron as it flows from the melting furnace or when it is received in the ladle from which the castings are to be poured.
The conversion is effected by a nodulariser introduced into the metal by means of a so-called nodularizing agent, for example, a nodularising alloy such as alloys of magnesium, calcium, lithium, strontium, barium, cerium, didynium, lanthanum and yttrium, which are readily oxidizable at the temperature of the molten cast iron or are volatile. The effect of these alloys, or nodularizing agents as they are generally termed herein, tends to disappear either during processing or subsequent standing and time taken in transferring the metal to the moulds, however, and as a result of this, there is a loss of strength properties as the carbon in the iron reverts to its original flake form. This effect is known as "fadin" and is a common problem in the industry.
A number of techniques have been applied to overcome this phenomenon, for example, by using a large quantity of nodularizing agent or by making a further addition of nodularizing agent during the pouring process. All of these techniques, however, constitute an increased cost in manufacture, since the nodularizing alloys are expensive and rarely are the added alloys more than 40% effective in relation to the amounts used. Furthermore, the use of excessive quantities of nodularizing alloys is deleterious in that it tends to give rise to the formation of oxides or silicates which become entrapped in the melt producing dirty castings or dross defects, and may also give rise to subsurface blow-holes and `elephant skin`. The presence of excess nodularizer in the metal may also intensify contraction of the molten iron during solidification giving rise to shrinkage and other defects and consequent loss of physical properties, and since most of the alkali and earth metals used as nodularizers, e.g. cerium, didynium, lithium, magnesium and strontium are also carbide stabilizers, there is an inherent problem of over treatment leading to the production of hard and brittle castings and consequent loss of machinability and ductility. All of these problems, of course, lead to a loss of confidence in the dependability of the finished product, and in the case of modern production line castings, the reliability and reproducibility of the casting process are of considerable economic significance.
In U.S. Pat. No. 3,703,922, a process was described for the treatment of molten cast iron for conversion of the graphite (carbon) content to nodular or spheroidal form, which provides the basis for efficient, and above all, reliable production of nodular cast iron.
The basic concept of the invention described in U.S. Pat. No. 3,703,922 is that the nodularizer addition is effected within or adjacent the mould block and so close to the mould cavity itself. Accordingly, provision is made in the mould for a pre-formed chamber termed the `intermediate chamber`, in addition to the conventional pouring bush, downsprue, gating and risering systems required to correctly introduce the molten metal to the casting cavity itself. The intermediate chamber contains the nodularizing alloy, and may be located in any suitable position relative to the pouring bush, downsprue, gating and risering system or the casting cavity itself, in such a manner that the incoming molten metal comes into contact with the nodularizing agent before entering the mould cavity.
By this means, reaction of the nodularizing agent (alloy) with the molten metal commences uniformly and continues progressively until the mould cavity is completely filled, provided that the nodularizing agent is not, of course, exhausted before this stage is reached. As a result of this progressive reaction, the nodularizer is taken up into the metal at a uniform rate, and the whole batch of molten metal is thus uniformly treated leading to constant properties throughout the resultant casting. By siting the point of treatment close to, even adjacent to the mould cavity, the problem of fading is overcome, and since the reaction occurs when the molten metal covers the nodularizing agent, it occurs out of contact with the air, so that losses due to volatilization and oxidation are completely eliminated. Undesirable pyrotechnics, fume or turbulence is also avoided and greater economy of treatment is, therefore, achieved by this process.
A further advantage of the process in U.S. Pat. No. 3,703,922 is that it provides broadly a process in which it is possible to control quite precisely the amount of nodularizer that is required, for a given weight of metal, to convert the graphite entirely from flake to nodular form, while at the same time avoiding over treatment, with its attendant problems such as dross inclusions referred to earlier.